1. Field of the Invention
This invention relates to rotary ball valves of the type employed in subterranean wells, and particularly to an apparatus for imposing a substantially constant loading between the ball and the annular seal which cooperates with the spherical ball surface.
2. Description of the Prior Art
A ball valve is one of the most popular types of valves employed in subterranean wells for the control of fluid passing through a conduit. Balls valves are found in safety valves and in test trees. For example, a pair of typical ball valves are described and illustrated in my co-pending application, Ser. No. 064,455 filed Aug. 6, 1979, now U.S. Pat. No. 4,306,623 entitled "Valve Assembly For A Subterranean Well Conduit", and assigned to the Assignee of this application.
The typical ball valve embodies a spherically shaped element having a cylindrical fluid passage through its center. An annular seal disposed transversely of a fluid conduit, cooperates with a spherical segment portion of the surface of the ball valve, particularly when it is shifted to its closed position wherein the fluid passageway through the valve is disposed transversely to the fluid conduit in which the ball valve is mounted.
Ball valves are customarily operated by a camming sleeve, or by a pair of diametrically opposed, camming sleeve segments, which incorporate cam slots which respectively receive radially projecting diametrically opposed pins projecting from the periphery of the ball valve and being offset from the rotational center of the ball. Axial movement of the camming sleeve or segments thus effects a 90.degree. rotation of the ball valve required to rotate it from a fully opened to a fully closed position.
In prior art valve assemblies, the loading of the ball valve against the annular seal is a matter of some uncertainty. It is common to employ an elastomeric material for the annular seal and then to impose the loading force through the camming pins when they are moved by the camming sleeves to their closed position. Since the mechanism for operating the camming sleeve is generally reasonably complicated and subjected to an accumulation of tolerances, it is understandable that, in prior valves, there are wide variations in the loading force imposed between the ball valve and the annular elastomeric seal. Excessive loads on the elastomeric material will inherently result in the tearing of the material when the ball valve is next rotated to its opened position. Too light loading results in leakage, particularly as the operating parts wear or corrode.